The combustion of fossil fuels in activities such as electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Research since the 1970s indicates increasing concentrations of carbon dioxide in the atmosphere may be responsible for altering the Earth's climate, changing the pH of the ocean and other potentially damaging effects. Countries around the world, including the United States, are seeking ways to mitigate emissions of carbon dioxide.
A mechanism for mitigating emissions is to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals. If the carbon dioxide is converted using energy from renewable sources, both mitigation of carbon dioxide emissions and conversion of renewable energy into a chemical form that can be stored for later use will be possible. Urea is an important fertilizer and industrial chemical used around the world. Industrially, urea is synthesized from carbon dioxide and ammonia at temperatures between 150 to 210 degrees Celsius and pressures of 120 to 400 atmospheres. Ammonia is typically produced from hydrogen and nitrogen at relatively high temperatures and pressures. The overall process of industrially synthesizing urea requires a large amount of energy, which generally comes from natural gas. The combustion of natural gas contributes to the concentration of carbon dioxide in the atmosphere and thus, global climate change.
Previous work in the field of electrochemical techniques has many limitations, including the stability of systems used in the process, the efficiency of systems, the selectivity of the systems or processes for a desired chemical, the cost of materials used in systems/processes, the ability to control the processes effectively, and the rate at which carbon dioxide is converted. In particular, existing electrochemical and photochemical processes/systems have one or more of the following problems that prevent commercialization on a large scale. Several processes utilize metals such as ruthenium or gold that are rare and expensive. In other processes, organic solvents were used that made scaling the process difficult because of the costs and availability of the solvents, such as dimethyl sulfoxide, acetonitrile and propylene carbonate. Copper, silver and gold have been found to reduce carbon dioxide to various products, however, the electrodes are quickly “poisoned” by undesirable reactions on the electrode and often cease to work in less than an hour. Similarly, gallium-based semiconductors reduce carbon dioxide, but rapidly dissolve in water. Many cathodes produce a mixture of organic products. For instance, copper produces a mixture of gases and liquids including carbon monoxide, methane, formic acid, ethylene, and ethanol. Such mixtures of products make extraction and purification of the products costly and can result in undesirable waste products that must be disposed. Much of the work done to date on carbon dioxide reduction is inefficient because of high electrical potentials utilized, low faradaic yields of desired products, and/or high pressure operation. The energy consumed for reducing carbon dioxide thus becomes prohibitive. Many conventional carbon dioxide reduction techniques have very low rates of reaction. For example, in order to provide economic feasibility, a commercial system currently may require densities in excess of 100 milliamperes per centimeter squared (mA/cm2), while rates achieved in the laboratory are orders of magnitude less.